1. Field of the Invention
The present invention relates to semiconductor devices. In particular, the present invention relates to semiconductor devices including oxide semiconductor.
2. Description of the Related Art
In recent years, techniques aiming to lower consumption of semiconductor devices have been developed actively with growing interest in energy resource.
The term “semiconductor device” in this specification indicates all the devices that operate by utilizing semiconductor characteristics. Such a semiconductor device includes transistors and for example, has functional circuits such as memories and processors including transistors.
As the semiconductor device, for example, there is a semiconductor device including a memory provided with a transistor serving as a memory element (also referred to as a memory transistor) (for example, see Patent Document 1).
In a semiconductor device disclosed in Patent Document 1, a memory transistor includes a control gate electrode, a channel formation layer, and a floating gate electrode provided between the control gate electrode and the channel formation layer. By accumulation of electrical charge to be data in the floating gate electrode, data is written to the memory transistor.
The conventional semiconductor device in Patent Document 1 has some problems of data loss due to electric charge leakage, high power consumption, and deterioration of a memory element.
Another example of the semiconductor devices is a semiconductor device including a memory using a selection transistor and an output transistor (for example, Patent Document 2).
In the semiconductor device described in Patent Document 2, the selection transistor is turned on to control electric charge accumulated in a gate of the output transistor, whereby data is written. After that, the selection transistor is turned off so that the electric charge accumulated in the gate of the output transistor is retained, whereby data is held.